The conventional Clark type oxygen sensor needs frequent recalibration when implanted in vivo due to membrane fouling, electrolyte thickness changes due to pressure variations, and the fluctuating flow rate of blood. An improved oxygen sensor is proposed (which is a modified Clark sensor) which reduces or eliminates many of those problems. The proposed sensor uses the following approaches: 1) a measuring pulse based on the establishment of a transient diffusional concentration gradient, 2) an electrical waveform for in situ conditioning (reactivating) of the sensor electrode, 3) all noble metal, non- contaminating electrodes in a non-chloride, acidic electrolyte, and 4) sensing electrode geometries which enhance the signal to noise ratio. During Phase I the optimum material and electrical parameters will be chosen based exclusively on in vitro laboratory testing of a macrosensor. Work during Phase II will concentrate on translating this technology into an implantable sensor for blood and/or tissue oxygen measurements and on carrying out an extensive in vivo testing program aimed at verifying the approach and at optimization of the sensor.